An Introduction to 700 Million Years of Earth History in Shropshire and Herefordshire
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ISSN 1750-855X (Print) ISSN 1750-8568 (Online) An introduction to 700 million years of earth history in Shropshire and Herefordshire 1 Peter Toghill TOGHILL, P. (2008). An introduction to 700 million years of earth history in Shropshire and Herefordshire. Proceedings of the Shropshire Geological Society, 13, 8–24. The beautiful landscape of the Welsh Marches is underlain by a rock sequence representing 10 of the 12 recognised periods of geological time. This remarkable variety, covering 700 million years of Earth history, has resulted from the interplay of three main factors: (1) erosion and faulting which have produced a very complex outcrop pattern; (2) southern Britain's position near to plate boundaries through most of late Precambrian and Phanerozoic time; and, most importantly, (3) the incredible 12,000 km, 500 million year, journey of southern Britain across the Earth's surface from the southern hemisphere to the northern, caused by plate tectonic processes. 1Church Stretton, Shropshire, UK. E-mail: [email protected] BACKGROUND This paper set the geological scene for the one-day symposium at Ludlow forming the centrepiece of the 2007 Marches Festival of Geology. However, this is not the place to provide a detailed description of the geology of Shropshire per se, for which the reader is referred to Peter Toghill’s Geology of Shropshire (2006), thereby to benefit from the author’s detailed knowledge of the local geology. The beautiful landscape of the Welsh Marches is underlain by a rock sequence representing ten of the twelve recognised periods of geological time (10 out of 13 if the Tertiary is subdivided into two periods). This remarkable variety, covering 700 million years of Earth history, has resulted from the interplay of three main factors: (1) erosion and faulting which have produced a very complex outcrop pattern; (2) southern Britain's position near to plate boundaries through most of late Precambrian and Phanerozoic time; and, most importantly, (3) the incredible 12,000 km, 500 million year, journey of southern Britain across the Figure 1. Geological map of Shropshire. Only the major Earth's surface from the southern hemisphere to faults are shown. Most information based on British the northern, caused by plate tectonic processes Geological Survey 1:250,000 Series Map, Solid Geology, (Figures 1, 2, 3 & 4). Mid-Wales and Marches, 1990, by permission of the British Geological Survey, copyright permit IPR/51-03C. Proceedings of the Shropshire Geological Society, 13, 8−24 8 © 2008 Shropshire Geological Society INTRODUCTION TO SHROPSHIRE AND HEREFORDSHIRE shows Shropshire’s incredible journey of 12,000 km in 500 million years. Redrawn with permission from data in Woodcock and Strachan, Geological History of Britain and Ireland, Blackwell Science, 2000, p.30, Fig. 2.7. Chapter 3. Figure 2. Structural Geology of Shropshire. Most information from British Geological Survey 1:250,000 Series Map, Solid Geology, Mid-Wales and Marches, 1990, by permission of the British Geological Survey, copyright permit IPR/51-03C. Figure 4. Sequence of sedimentary and volcanic rocks in Shropshire (“stratigraphic column”) and main periods of earth movements. Intrusive igneous rocks not shown. © Copyright 2006 Peter Toghill Figure 3. Movement through latitudes of the continents and micro continents that included parts of Britain, from late Precambrian to the present day. The orange line Proceedings of the Shropshire Geological Society, 13, 8−24 9 © 2008 Shropshire Geological Society P. TOGHILL The story begins on the northern margins of years ago (Ma) to form a marginal basin bounded Gondwana, around 60 degrees south of the equator by faults now called the Welsh Borderland Fault ca. 600 million years ago (Figures 5 & 6). System. The most famous of these faults, the Church Stretton Fault, together with its partner further west, the Pontesford-Linley fault, would have a profound affect on Shropshire geology for nearly 500 million years (Figure 7). Figure 5. Late Precambrian palaeogeography. (a) Vendian supercontinent showing positions of Cadomian Volcanic Arc and continental split from Iapetus Ocean; (b) formation of early Iapetus Ocean. Redrawn with permission from data in Woodcock and Strachan, Geological History of Britain and Ireland, Blackwell Science, 2000, p.25 and 26, Figs. 2.4b and 2.5a. Figure 7. Late Precambrian Welsh Borderland Marginal Basin showing formation of Uriconian Volcanics and Longmyndian Supergroup. The late Uriconian and early Longmyndian (Stretton Group) are probably of the same age. © Copyright 2006 Peter Toghill A volcanic arc here, near to the margins of the basin, formed the Uriconian Volcanics of Shropshire between 570 and 560 Ma, and around the same time a nearby shallow marine basin received sediments from the eroding volcanic arc and adjacent areas to form the unique Longmyndian Supergroup of sedimentary rocks, up to 7,000 m thick. Figure 6. Late Precambrian plate tectonics. Formation of The marine basin rapidly shallowed so that the Rushton Schists and Primrose Hill Gneisses and Schists, and formation of pull-apart marginal basin bounded by early Longmyndian fine grained marine and the Welsh Borderland Fault System including Church deltaic/fluvial sequence, the Stretton Group, is Stretton and Pontesford-Linley Faults. © Copyright 2006 followed, without an unconformity, by the coarse Peter Toghill grained fluvial Wentnor Group. Major earth movements around 550 Ma (latest A late Precambrian basement of metamorphic Precambrian) formed the remarkable overturned rocks and igneous complexes, now exposed as the Longmynd Syncline, exposed around Church Rushton Schists of Shropshire and the igneous Stretton between the Church Stretton and rocks of the Hanter and Stanner Hills in Pontesford-Linley Faults, with small exposures Herefordshire, was split apart around 600 million further south around Old Radnor. The compression Proceedings of the Shropshire Geological Society, 13, 8−24 10 © 2008 Shropshire Geological Society INTRODUCTION TO SHROPSHIRE AND HEREFORDSHIRE was mainly transpressional, with considerable lateral movements along the Pontesford-Linley Fault and Church Stretton Fault, and it is possible that the Longmyndian is a displaced terrane (Figures 8 to 22). Figure 10. Outcrop of Shropshire’s Precambrian rocks; Section A-B is shown in Figure 9. © Copyright 2006 Peter Toghill Figure 8. Deformation of Uriconian and Longmyndian during the late Precambrian Avalonian orogeny. © Copyright 2006 Peter Toghill Figure 11. Rushton Schist poorly exposed in a rutted track. Wrekin in the background (long ridge); Little (Primrose) Hill is the small “bump” to its right. © Figure 9. Long Mynd Syncline approximately along the Copyright 2006 Peter Toghill line A-B in Figure 10. Present-day relationships of Longmyndian and Uriconian. PLF, Pontesford-Linley Fault; RC, Radlith Conglomerate; OC, Oakswood Conglomerate; LC, Lawnhill Conglomerate; SC, Stanbatch Conglomerate; DC, Darnford Conglomerate; HC, Haughmond Conglomerate; HUC, Huckster Conglomerate; BV, Batch Volcanics; CG, Cardingmill Grit; BR, Buxton Rock; HG, Helmeth Grit; CSF, Church Stretton Fault. © Copyright 2006 Peter Toghill Proceedings of the Shropshire Geological Society, 13, 8−24 11 © 2008 Shropshire Geological Society P. TOGHILL Figure 12. Church Stretton Valley from the Long Mynd Figure 15. Finely laminated, water-lain, Ragleth Tuffs on looking towards Caer Caradoc and the Lawley, with the the flanks of Caer Caradoc. © Copyright 2006 Peter Wrekin in the distance. The main Church Stretton Fault is Toghill one-third of the way up the side of Caer Caradoc. Church Stretton Valley has only one main fault along it and is not a rift valley. This fault could be a listric fault. © Copyright 2006 Peter Toghill Figure 16. The Battlestones. A rhyolitic crag at the northern end of Willstone Hill with Willstone Hill Conglomerates (? Wentnor Group Longmyndian) to the left of the crag. Caer Caradoc in the distance. © Figure 13. In the foreground: Uriconian andesites on the Copyright 2006 Peter Toghill Lawley; looking south towards Caer Caradoc and the Long Mynd (beyond). Despite the conical shape of Caer Caradoc, it is not a volcanic cone but a remnant of a wider extent of Uriconian lavas and ashes – no vents have been found within the Uriconian rocks of Shropshire. © Copyright 2006 Peter Toghill Figure 17. The Long Mynd from the Gaer Stone. © Copyright 2006 Peter Toghill Figure 14. Uriconian rhyolitic ash flow and andesitic ashes at Leaton Quarry, Telford. © Copyright 2006 Peter Toghill Proceedings of the Shropshire Geological Society, 13, 8−24 12 © 2008 Shropshire Geological Society INTRODUCTION TO SHROPSHIRE AND HEREFORDSHIRE Figure 20. Cleaved purple shales in the Synalds Formation, Longmyndian, Haddon Hill, Long Mynd. The beds are dipping steeply to the left whereas the cleavage is just about vertical, parallel to the ski stick. © Copyright 2006 Peter Toghill Figure 18. The Longmyndian sequence showing environments of deposition and types of sediment. After Pauley, 1990, Sedimentology, Structural Evolution and Tectonic Setting of the Late Precambrian Longmyndian Supergroup of the Welsh Borderland, p.345, Fig.3. In D’Leoms et al., 1990, The Cadomian Orogeny. Figure 21. Possible fossil rain prints? Synalds Formation, Geological Society Special Publication No.51, pp. 341- Longmyndian. © Copyright 2006 Peter Toghill 351. Figure 22. Darnford Conglomerate in the Bayston- Figure 19. Burway Formation turbidites, Burway Hill, Oakswood Formation, Wentnor Group, Longmyndian.